netdev - Re: [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is never 0

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Message-ID: <CAEf4BzZCK5L-yZHL=yhGir71t=kkhAn5yN07Vxs2+VizvwF3QQ@mail.gmail.com>
Date:   Tue, 5 Oct 2021 21:41:29 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v1 3/6] libbpf: Ensure that module BTF fd is
 never 0

On Tue, Oct 5, 2021 at 5:29 PM Kumar Kartikeya Dwivedi <memxor@...il.com> wrote:
>
> Since the code assumes in various places that BTF fd for modules is
> never 0, if we end up getting fd as 0, obtain a new fd > 0. Even though
> fd 0 being free for allocation is usually an application error, it is
> still possible that we end up getting fd 0 if the application explicitly
> closes its stdin. Deal with this by getting a new fd using dup and
> closing fd 0.
>
> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@...il.com>
> ---
>  tools/lib/bpf/libbpf.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index d286dec73b5f..3e5e460fe63e 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -4975,6 +4975,20 @@ static int load_module_btfs(struct bpf_object *obj)
>                         pr_warn("failed to get BTF object #%d FD: %d\n", id, err);
>                         return err;
>                 }
> +               /* Make sure module BTF fd is never 0, as kernel depends on it
> +                * being > 0 to distinguish between vmlinux and module BTFs,
> +                * e.g. for BPF_PSEUDO_BTF_ID ld_imm64 insns (ksyms).
> +                */
> +               if (!fd) {
> +                       fd = dup(0);

This is not the only place where we make assumptions that fd > 0 but
technically can get fd == 0. Instead of doing such a check in every
such place, would it be possible to open (cheaply) some FD (/dev/null
or whatever, don't know what's the best file to open), if we detect
that FD == 0 is not allocated? Can we detect that fd 0 is not
allocated?

Doing something like that in bpf_object__open() or bpf_object__load()
would make everything much simpler and we'll have a guarantee that fd
== 0 is not going to be allocated (unless someone accidentally or not
accidentally does close(0), but that's entirely different story).

> +                       if (fd < 0) {
> +                               err = -errno;
> +                               pr_warn("failed to dup BTF object #%d FD 0 to FD > 0: %d\n", id, err);
> +                               close(0);
> +                               return err;
> +                       }
> +                       close(0);
> +               }
>
>                 len = sizeof(info);
>                 memset(&info, 0, sizeof(info));
> --
> 2.33.0
>